1. Field of the Invention
The present invention relates to a battery charger for use with a chargeable battery such as a Ni--Cd battery.
2. Description of the Prior Art
Generally, when a battery is charged, its voltage varies with time as indicated by a solid line in FIG. 1. Thus, the battery voltage gradually increases until the battery is fully charged. After the battery has been fully charged, the battery voltage decreases. This means that the battery voltage becomes maximum when the battery has been fully charged. Based on this fact, the prior art has proposed a technique known as -dV method in which, by monitoring the battery voltage during the charging operation, the charging operation is stopped when the battery voltage has been dropped by a predetermined value after reaching the maximum value. Such a technique is disclosed in Japanese Laid-Open Patent Publication Nos. 54-118542 and 55-141938.
However, when an over-discharged battery is charged, the battery voltage varies as indicated by a dashed line A in FIG. 5. Thus, in this case, the battery voltage once abruptly increases immediately after starting the charging operation and then abruptly decreases (see line parts A4 and A6). The battery voltage thereafter gradually decreases (see line part A2) and subsequently increases in the same manner as a normal charging operation (see line part AS). Therefore, if the -dV method is unconditionally adopted for controlling the operation, in case of the charging of the over-discharged battery, the charging operation may be stopped in the course of the temporary abrupt variation of the battery voltage produced immediately after starting the charging operation. For this reason, various techniques have been proposed to prevent an occasion such that the charging operation is accidentally stopped by the abrupt variation produced immediately after starting the charging operation.
Japanese Laid-Open Patent Publication No. 61-288740 proposes a technique to inhibit determination of -dV unless the battery voltage begins to stably increase. Japanese Laid-Open Patent Publication No. 64-47232 or No. 4-58471 proposes technique in which determination of -dV is inhibited as long as the absolute value of the battery voltage is less than a predetermined value, so that the charging operation for the over-discharged battery may not be stopped so long as the battery is short of charge. Here, the predetermined value is selected to a voltage value which may be reached after the battery voltage has begun to stably increase.
As described above, in order to prevent the charging operation from being stopped by the abrupt variation produced immediately after starting the charging operation of the over-charged battery, the prior art techniques have operated to substantially invalidate the determination of -dV until the voltage of the over-discharged battery begins to stably increase or until timing A1 shown in FIG. 5.
However, in case that the determination of -dV is substantially invalidated until the battery begins to stably increase, the charging operation is continued until the inhibition period of determination of -dV is finished even if the battery is an already charged one and has been again charged accidentally. This may cause excessive charge of the battery, and it is not possible to prevent such excessive charge of the battery in case of adoption of this technique.
The stoppage of the charging operation during the abrupt variation produced immediately after starting the charging operation may be prevented if a reference value for the determination of -dV is larger than the value of the abrupt variation produced immediately after the charging operation. However, in this case, the charging operation may be continued for a significant time after the properly-charged battery has been fully charged.